The invention relates generally to semiconductor waveguide devices in integrated optics and more particularly to the electrode configuration in a semiconductor waveguide device.
Integrated optics utilize light transmission in optical waveguides, structures that confine propagating light to a region with at least one very small dimension. By the resultant miniaturization of individual devices an optical system can be formed on a small substrate. Integrated optics can be used for optical communications.
Waveguides can be made from many different materials utilizing many different techniques. Electro-optic modulators are utilized to impress information on the guided light wave. Some integrated optics modulators function on the principle that an electric field in certain materials produces changes in indices of refraction. Another type of modulator operates on the principle of the directional coupler used in microwave systems by electrically controlling the switching of the guided wave from one channel waveguide to a parallel and identical channel.
A semiconductor heterostructure consists of two different semiconductor materials in junction contact. A heterojunction is a junction in a single crystal between two dissimilar conductors. The two semiconductors differ in energy gap and refractive index. Differences in energy gap permit spatial confinement of injected electrons and holes while differences in refractive index are utilized to form optical waveguides.
The GaAs and Al.sub.x Ga.sub.1-x As heterostructures have been widely studied. Other III-V and II-VI systems can also be utilized. Materials can be doped either n-or p-type. Semiconductor heterostructures are usually fabricated as single crystal structures using thin film epitaxial crystal growth techniques such as liquid phase epitaxy, chemical vapor deposition and molecular beam epitaxy.
Various semiconductor electro-optic waveguides useful as switches or modulators have been developed, for example, "GaAs Electro-Optic Directional-Coupler Switch", by J. C. Campbell et al., Applied Physics Letters, Vol. 27, No. 4, Aug. 15, 1975, pg. 202; "Low-Loss GaAs p.sup.+ n.sup.- n.sup.+ Three-Dimensional Optical Waveguides", by F. J. Leonberger et al., Applied Physics Letters, Vol. 28, No. 10, May 15, 1976, pg. 616; "GaAs Directional-Coupler Switch with Stepped .alpha..beta. Reversal", by F. J. Leonberger et al., Applied Physics Letters, Vol. 31, No. 3, Aug. 1, 1977, pg. 223; "GaAlAs Schottky Directional-Coupler Switch", by A. R. Reisinger et al., Applied Physics Letters, Vol. 31, No. 12, Dec. 15, 1977, pg. 836; "Directional Coupler Switch in Molecular-Beam Epitaxy GaAs", by A. Carenco et al., Applied Physics Letters, Vol. 34, No. 11, June 1, 1979, pg. 755.
In waveguide electro-optic devices using semiconductors such as GaAs, applying bipolar fields to the electrodes has not been possible. The prior art devices cited above all utilize an electrode mounted on the top surface of tne waveguide structure and an ohmic contact mounted on the opposing face (bottom surface) of the substrate to apply an electric field to the device to introduce phase shifts through the electro-optic effect, i.e., only single rectifying contacts or junctions are utilized. With a single rectifying contact or junction it becomes necessary to apply unipolar signals to the electrodes; otherwise, the rectifying contact or junction draws a large current and cannot sustain a large voltage. The substrate has an electrical connection. Electric fields also cannot be applied transverse to the waveguide which is necessary for minimizing optical insertion losses. The limitation of applying only unipolar signals to the waveguide results in higher voltage and power requirements on the voltage source.